(1) Field of the Invention
The present invention relates to optical print heads and image forming devices, and in particular to technology implementing high image quality optical writing at low cost.
(2) Description of the Related Art
In recent years, an optical print head (PH) using organic light emitting diodes (OLEDs) has been proposed with an aim of reducing size and cost of an image forming device. The OLEDs are arranged in a line along a main scanning direction on a thin film transistor (TFT) substrate, and are electrically connected in parallel by power supply wiring provided along the main scanning direction (FIG. 16).
OLEDs are also called organic electro-luminescence (EL) elements. OLEDs are current-driven light-emitting elements, and when drive current is supplied to the OLEDs via the power supply wiring, a voltage drop occurs along the power supply wiring according to wiring resistance.
Drive transistors generating drive current for the OLEDs are arranged adjacent to the OLEDs in a one-to-one correspondence, and each drive transistor generates drive current according to a potential difference between potential at a connection of the power supply wiring and input voltage (luminance signal) from a digital to analogue converter (DAC). Thus, voltage drop of the power supply wiring leads to a decrease of reference potential, causing fluctuations in drive current of the OLEDs, and therefore emitted luminance fluctuates and uneven concentration occurs (FIG. 17A and FIG. 17B).
With respect to this problem, technology has been proposed to cause a capacitor to hold a source-gate voltage (hereafter, “gate voltage”) occurring due to drive current corresponding to luminance and an equal current energizing a source-drain of a drive transistor. In this way, drive current according to the voltage (luminance signal) held in the capacitor is supplied, and potential at the connection of the power supply wiring need not be the reference potential. Accordingly, a desired drive current is supplied to the OLEDs regardless of voltage drop of the power supply wiring, and uneven concentration can be eliminated.
Human vision is not sensitive to uneven luminance in moving images, and therefore, among monitor devices such as liquid crystal display monitors and OLED monitors, luminance unevenness up to 30% is allowable in a monitor screen. Further, in existing monitor devices, 256 grades for each color channel suffices, and therefore 8-bit DACs are sufficient. The reason for such a high tolerance for luminance unevenness and the low number of bits for DACs is that cost of monitor devices can be reduced.
In the case of OLED print heads, the dynamic range of luminance is 300%. Further, human sight is sensitive to uneven brightness in still images, and therefore an allowable amount of luminance unevenness is only a few percent. Thus, for example, in a case in which luminance unevenness is to be suppressed to no greater than 3%, the following is considered.
In order that luminance unevenness is no greater than 3%, resolution of luminance control must be one in ten of 3%, i.e. no greater than 0.3%. Further, for luminance control of a dynamic range of 300% at a resolution of 0.3%, luminance control must be performed at 300%÷0.3%=1000 grades for each color channel. Thus, 10-bit DACs are required for the conventional technology above.
Of course, as the number of bits for DACs increases, costs increase proportionately, and therefore increased costs are unavoidable when applying the conventional technology above.